the reason for the number of star points.

Folks describing interference at edge of aperture are right, Light spreads out (diffracts) perpendicular to the edges of the blades, with star spikes at right angles to flat/straight edges of the aperture opening.

So (if you really care) a 5-blade iris will give a 10-point sun star, a symmetical 6 blade iris a 6-point star (because opposite edges are parallel, their points overlap), 7-blade iris a 14-point star, etc...And this effect may be less pronounced/more diffuse with rounded iris touted on some newer lenses